The transmission modulator on a 1991 Cadillac Sedan DeVille is located
on the transmission itself. It's a small, usually cylindrical, valve body component connected to the transmission via a vacuum line. You'll find it somewhere on the side or top of the transmission case, but its precise location will vary slightly depending on the transmission type (e.g., 4T60-E).
Because of its location, accessing it requires getting underneath the vehicle. You'll likely need to consult a repair manual or a diagram specific to your 1991 Cadillac Sedan DeVille's transmission to pinpoint its exact position. These diagrams often show the vacuum lines connected to it, which can help in identification.
